While the vast majority of gearing solutions have used mechanical gearing arrangements, there is increasing interest in and demand for smaller, more lightweight, more efficient and less expensive gearing solutions that offer the high-torque transmission of existing mechanical arrangements. Magnetic gearing arrangements are an example of such a solution.
A magnetic gear uses magnetic fields to transmit torque without mechanical contact. In one form, a magnetic gear has three principle components, all three of which may rotate relative to each other. In one existing arrangement a radially inner one of the three components generates a first magnetic field with a first number of pole pairs. A radially outer one of the three components generates a second magnetic field with a second number of pole pairs. A radially intermediate one of the three components does not generate its own magnetic field. Instead, it has a number of ferromagnetic pole pieces supported by a non-magnetic and non-conductive structure. This third component acts as a passive part of a magnetic circuit between the first and second components. The role of the pole pieces is to modulate the first and second magnetic fields such that they interact in a geared manner. Consequently, torque can be transmitted between any two of the three components in a geared manner, or between all three of them in a manner similar to an epicyclic mechanical gear arrangement.
Other forms of magnetically geared apparatus comprise a passive gear with two permanent magnet arrays and a modulating ring; a motor generator with a stator wrapped around a magnetic gear; a motor generator with an integrated gear with a rotating permanent magnet rotor, a rotating modulating rotor, and a static array of magnets and windings; a variable magnetic gear with three rotors, two with permanent magnet arrays and a modulating rotor, and a stator winding to control the rotation of one of the rotors; and/or a variable magnetic gear with one permanent magnet rotor, a modulating rotor, and a stator winding which can couple with the modulated field and control rotational speed and therefore gear ratio.
An example of a magnetic gear is shown in FIG. 1. In this case, the outer component is additionally provided with a set of windings to become—in effect—a motor-generator. This arrangement combines the functionality of a magnetic gear and a typical electrical machine by allowing for geared torque transmission in combination with operation in either motoring or generating modes. In this case, the first and second magnetic fields are generated by permanent magnets on the inner and outer components. When the windings of the outer component are supplied with a three phase, 120 degree displaced current, a rotating magnetic field is set up in the machine. This rotating magnetic field has the same number of pole pairs as the first magnetic field generated by the inner component. The rotating magnetic field and the first magnetic field directly couple such that the harmonic of the first magnetic field can be used for electromechanical energy conversion.
The magnetic flux associated with the parts of the magnetic fields in the machine that interact and contribute to the transmission of torque is termed the ‘useful’ magnetic flux. However, not all of the flux is useful. Some of the magnetic flux does not contribute to the transmission of torque and instead propagates in a direction perpendicular to that of the useful magnetic flux. This is termed the ‘stray’ magnetic flux.
In order to accommodate the radially intermediate component, the magnetic gear must have an air gap between the other two components of the machine. Because this air gap must accommodate the intermediate component, it is much larger than the air gap between the two components of a conventional electrical machine. This arrangement can lead to high levels of stray magnetic flux in the air gap. This creates two key problems: first, a proportion of the magnetic field set up in the machine is wasted as it is not being used to transmit torque; and second, the stray magnetic field induces eddy currents in the pole pieces in addition to those induced by the useful magnetic flux. Both of these problems are sources of inefficiency in the machine. While all electrical machines can suffer from these problems, those based on magnetic gear technology are particularly vulnerable as the problems are exaggerated by the size of the air gap and the high frequency of the magnetic fields employed.
Eddy currents are undesirable for a number of reasons. First, eddy currents induce a ‘secondary’ magnetic field which opposes the ‘primary’ magnetic field that created it. The resultant field (that is, the sum of the primary and secondary magnetic fields) is weaker than the primary magnetic field. In the case where the primary magnetic field is the useful magnetic field, this reduces the efficiency of the machine. Second, the eddy currents have a heating effect in the pole pieces due to resistive losses. Undesirably, this is a form of energy loss in the machine. The additional heating can also damage the mechanical properties of the pole piece support structure and of other materials in the machine and undesirably can cause the temperature in other parts of the machine to increase. The overall decrease in efficiency of the machine caused by eddy currents manifests itself either as a decrease in torque at the output shaft, or an increase in drag torque at the drive shaft.
These problems can be addressed in part by manufacturing the pole pieces from stacks of thin magnetically conductive sheets, or laminations, which are electrically insulated from each other. An example of such a laminated pole piece is shown in FIG. 2. The laminations act to restrict the flow of the eddy currents induced by the useful magnetic flux. The laminations are orientated such that the laminations lie parallel with the lines of useful magnetic flux. This orientation is adopted since the eddy currents induced by the useful magnetic flux are a greater source of loss in the machine than the eddy currents induced by the stray magnetic flux. Laminating the pole pieces in this way does not however restrict the flow of eddy currents induced by the stray magnetic flux. The eddy currents induced by the stray magnetic flux continue to circulate within the planes of the individual laminations, as shown in FIG. 3. As a result, the flow of eddy currents induced by the stray magnetic flux remains a source of loss in the machine.
Alternative solutions for at least restricting the flow of eddy currents include manufacturing the pole pieces from sintered or composite soft magnetic materials. However this can be problematic as this can significantly reduce the mechanical strength of the pole pieces.
Accordingly, it is an object of at least some of embodiments of this invention to address these problems.